1. Field of the Invention
This invention relates to a method and apparatus for driving a piezoelectric transformer and particularly relates to the method and apparatus for driving the piezoelectric transformer as a high voltage power source for a copier and a printer utilizing an electrophotographic system.
This application is based on Patent Application No. Hei 9-300237 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
As a conventional method and apparatus for driving a piezoelectric transformer, a method and apparatus for driving a piezoelectric transformer used for an inverter power supply of a cold cathode tube is disclosed in Japanese Patent Application No. Hei 9-135573. In this inverter power supply of the cold cathode tube, a piezoelectric transformer is used as a boosting transformer for the inverter power supply for a cold cathode fluorescent lamp for use as a back-light of a liquid-crystal display. In this application, a pulse-width variation means is adopted as the method of driving the piezoelectric transformer for safe lighting of the cold cathode fluorescent lamp in response to the changes of the source voltage and the load.
FIG. 10 is a block-diagram showing a conventional driving apparatus of a piezoelectric transformer. In FIG. 10, a feedback resistor 76 with a small resistance is connected to a cold cathode fluorescent lamp 75 in series, and current which flows in the cold cathode fluorescent lamp is detected by the feedback resistor 76. The voltage applied across both ends of the feedback resistor is input into a current detection circuit 77 and a phase detection circuit 78. The current detection circuit 77 detects a load current which flows from the voltage at both ends of the feed back resistance 76 to the cold cathode fluorescent lamp 75 and the result of the detection is input into the pulse width control circuit 79.
The pulse width control circuit 79 controls a pulse width by means of a pulse width variable circuit 80, so as to stabilize the voltage at both ends of the resistance at a constant value, that is , so as to stabilize the load current flowing in the cold cathode fluorescent lamp at a constant value. A pulse signal output from the pulse width variable circuit 80 is taken by a waveform shaping circuit 72 at an amplitude corresponding to the pulse width, which is converted into a driving signal by the drive circuit 73 and which is supplied to the piezoelectric transformer 74 to drive thereof. The piezolectric transformer boosts the signal and the boosted signal is supplied to the cold cathode fluorescent lamp.
Here, if, for example, a current flowing in the cold cathode fluorescent lamp 75, which corresponds to a load, becomes small, the voltage at both ends of the feedback resistor becomes small so that the pulse width control circuit 79 detects that the voltage at both ends of the feedback resistor is lower than a predetermined value. An instruction is then transmitted to the pulse width variable circuit 80 so as to expand the pulse width, and the waveform shaping circuits 72 expands the amplitude of the output signal. Thereby, the input to the piezoelectric transformer 74 is increased, and the current flowing in the cold cathode fluorescent lamp is also increased.
In contrast, when the current of the cold cathode fluorescent lamp becomes large, the voltage at both ends of the feedback resistor 76 increases. The pulse-width control circuit 79 detects that the voltage at both ends of the feedback resistor 76 is higher than a predetermined value and transmits an instruction to the pulse-width control circuit 72, and an amplitude of the output signal of the waveform shaping circuit 72 is reduced. Thereby, the input of the piezoelectric transformer 74 is reduced and the electric current flowing in the cold cathode fluorescent lamp is reduced.
The above control system enables the stabilization of the current in the cold cathode fluorescent lamp at a constant value so that the lamp is lighted at a constant brightness. In addition, the phase detection circuit 78 sets a limitation in the relationship between the resonance frequency and the driving frequency.
However, when the impedance of the cold cathode fluorescent lamp becomes extremely low, this conventional driving apparatus must reduce an output voltage of the piezoelectric transformer. When the output voltage is reduced, the ratio of higher harmonic wave components in the driving signal becomes quite high, so that the loss in the piezoelectric transformer 74 becomes high, the driving efficiency of the piezoelectric transformer 74 becomes low, and the reliability also becomes low.
FIG. 11 is a block-diagram showing an example of a conventional piezoelectric transformer which is improved so as to solve the above problems. In the figure, the same constituting elements as those in FIG. 10 are represented by the same reference numerals and the explanations of those elements are omitted. In FIG. 11, when the impedance of the cold cathode fluorescent lamp 75 becomes extremely low, the output current is too high even though the driving voltage is constant; so that the pulse width control circuits 81 outputs an instruction to the pulse width variable circuit 80 so as to reduce the pulse width of the output signal, and at the same time, an instruction is outputted to the DC-DC converter 82. The DC-DC converter 82, after receiving a detected voltage of the source voltage from a source voltage detection circuit 83, reduces the output current voltage of the DC-DC converter 82 based on the instruction. The reduced current voltage is supplied to the driving circuit as the DC source. This causes the restoration of the pulse width of the driving signal supplied to the piezoelectric transformer 74 from the driving circuit 84 into a normal width.
In contrast, when the impedance of the cold cathode fluorescent lamp 75 becomes quite high, a sufficient output current may not be taken out, even if the pulse width of the driving voltage of the piezoelectric transformer 74 is expanded. In this case, the pulse width control circuit 81 supplies an instruction so as to expand the pulse width, and at the same time, an instruction to raise the output current voltage is supplied to the DC-DC converter 82. The thus raised current voltage is supplied to the driving circuit 84 as the DC source for controlling the pulse width of the output driving current at the expanded state. Thus, the conventional circuit is designed such that conversion of the DC voltage is executed by DC-DC converter 82 only when the pulse width of the driving signal is outside of a predetermined range.
However, the variable control of the pulse width of the driving signal by the conventional apparatus may possibly lose a fixed relationship between the driving frequency and the resonance frequency of the piezoelectric transformer. When the driving frequency is far away from the resonance frequency of the piezoelectric transformer 74, the driving efficiency of the piezoelectric transformer 74 may be remarkably reduced or only a variable control of the pulse width may not allow for sufficient current to flow in the cold cathode fluorescent lamp 75 as a load.
FIG. 12 shows a block-diagram of an example of a conventional driving apparatus of the piezoelectric transformer which was constructed to overcome the above problem. In the figure, the same constituting elements as those in FIGS. 8 and 9 are represented by the same reference numerals and explanations of those elements are omitted. In the conventional driving apparatus shown as a block diagram in FIG. 12, the driving frequency is rendered variable within a limited range by setting a limit in the relationship between the resonance frequency and the driving frequency of the piezoelectric transformer 74 by the phase detection circuit 78; and also by setting a limitation in the relationship between the resonance frequency and the driving frequency of the piezoelectric transformer 74 by inputting the output of the current detection circuit 85 into the variable oscillation circuit 86.
Accordingly, when the output current becomes low even if the phase difference is within a specified range, the above construction permits the pulse width control circuit 79 to supply an instruction to the pulse width variable circuit 80 so as to expand the pulse width of the driving signal to enlarge the amplitude of the output signal of the waveform shaping circuit 72. Furthermore, if the output current is still smaller than a prescribed value even though the pulse width ofthe.pulse signal is reduced, the output current is increased by reducing the driving frequency within a prescribed frequency range corresponding to a prescribed range of the phase difference.
Conversely, if the load current is too large, the pulse width control circuits 79 may supply an instruction to the pulse width variable circuit 80 so as to reduce the pulse width of the driving signal to reduce the amplitude of the output signal of the waveform shaping circuit 72. In addition, if the output current is still larger than the specified value even though the pulse width of the driving signal is reduced, the output current is further reduced by increasing the driving frequency within a driving frequency range corresponding to the set range of the phase difference.
A conventional art as to setting a hysteresis character is disclosed, for example, in Japanese Patent Application, First Publication Hei 5-183524. In the above application, two different values are used, that is, a hysteresis is given, as reference values for comparing two output voltages at the time of switching a step-attenuator of an IF band signal of a signal receiving apparatus using an optical transmission system. Another example is disclosed in Japanese Patent Application, First Publication No. Hei 3-241920, in which two different values are provided, or a hysteresis character is given for changing output voltages of an standard voltage source for different input voltages to a reset circuit. Moreover, Japanese Patent Application, First Publication No. Hei 1-235414 discloses to set two different values, that is, to give a hysteresis between an operating point at which the MOS-type field effect transistor (MOS FET) is switched from on to off, and an operating point at which MOS FET is switched from off to on.
Whereas, the conventional method and apparatus for driving the piezoelectric transformer has a problem that the output voltage does not have a wide variable range. The reason for this is because the output voltage is varied by changing the boosting ratio of the piezoelectric transformer 74 by changing the driving frequency of the piezoelectric transformer 74.
Since it is preferable to drive the piezoelectric transformer at high efficiency by use of a sine wave which does not include any unnecessary frequency component, the driving waveform is adjusted so as to be shaped in a sine wave utilizing a voltage resonance. The driving circuit 73 utilizes a voltage resonance by an inductance and a capacitance, and half sine waves are produced and a sine wave is produced by combining two half waves by executing the optimum E class operation at the zero voltage switching at the time of turning the switch on. A coil or an electromagnetic transformer are used for the inductance, and an equivalent input capacity of the piezolelectric transformer is used for the capacitance.
The driving waveform of the thus constructed driving apparatus is tend to change by the driving frequency, and it is hard to obtain a sine wave, when the driving frequency is out of the frequency used for the circuit adjustment and the efficiency of the piezolelectric transformer decreases. Particularly, when a higher frequency than that used for the circuit adjustment is used, the driving waveform is changed and the efficiency is reduced. The driving frequency from which the efficiency begin to decrease is a natural frequency determined by the vibration mode, the element structure, the element size, and the driving circuit system of the piezoelectric transformer.
When the driving frequency reaches the natural frequency which is higher than the frequency used for the circuit adjustment, the driving waveform is broken due to the collapse of the voltage resonance to form the half sine wave lacking the latter half. If the driving frequency is higher than the natural frequency, the waveform becomes a separated sector form since the break of the half sine wave extends further. Although the driving apparatus is maintained at high efficiency by zero voltage switching, if the driving waveform becomes the sector form, the switching is rendered to the non-zero switching and the driving circuit is heated. Since the potential of non-zero switching is proportional to the degree of breakage, so that the larger the breakage, the higher the heating. The heat generation of the piezoelectric transformer and the driving apparatus reduces the efficiency of the power supply and finally the operation is deteriorated, so that the upper limit to be able to shift the driving frequency is the natural value. It is necessary for the high voltage power supply to provide a wide frequency range to be able to move the frequency range for obtaining a wide output voltage range. However, the upper limit of the driving frequency is limited by the natural frequency, so that a wide variable frequency range is not obtainable.
A conventional method and apparatus for driving piezoelectric transformers is disclosed in Japanese Patent Application, First Publication No. Hei 9-135573, which is used for fixing the load current flowing in a cold cathode fluorescent lamp irrespective of the ambient circumstances. The above disclosed method first detects a difference in phases between the input and output power by a phase difference detection circuit 78, and while variably controlling the pulse width, the method restricts the fluctuating frequency width of the driving frequency by the phase difference. However, the above method is not effective when the natural frequency is included within a frequency width restricted by the phase difference.
The second problem is that driving waveform distortion is generated when the upper limit of the voltage of the power supply is expanded and causes a problem in reliability. The reason for this is that, when the amplitude of the driving waveform increases by the increase of the voltage of the power supply, the driving frequency shifts to higher frequency than the resonance frequency for maintaining a fixed output power.
The driving waveform is, utilizing a voltage resonance of the inductance and the capacitance, produced by combining two half sine waves formed by executing the optimum E class operation which corresponds to a zero voltage switching when the switch is turned on. A coil or an electromagnetic transformer is used as the inductance, and an equivalent input capacity of the piezoelectric transformer is used as the capacitance. Thus, when the voltage of the power supply increases, then the input voltage to the driving circuit is increased.
The amplitude of the half sine wave produced by the voltage resonance increases in proportion to the input voltage, and, therewith, the amplitude of the sine wave which corresponds to the driving waveform increases. When the amplitude of the driving waveform increases, a control to reduce the boosting ratio of the piezoelectric transformer for maintaining the output power at a fixed value begins. The boosting ratio of the piezoelectric transformer has a characteristic which forms a convex curve against the driving frequency with a peak at the resonance frequency. Therefore, since the boosting ratio is reduced when the driving frequency is made higher than the resonance frequency, a control starts to drive at higher frequency than the resonance frequency for maintaining a constant output power if the amplitude of the driving waveform increases.
However, if the piezoelectric transformer is driven at higher frequency than the resonance frequency, distortion of the driving waveform is generated by superimposing higher harmonic waves of the driving frequency on the natural frequency which is determined by the vibration mode, the element structure, the element size, and the driving circuit construction of the piezoelectric transformer. When the upper limit of the source voltage is extended wider, the driving frequency is made to use a higher frequency range so that there is a higher possibility for the driving frequency to be operated at the natural frequency.
Moreover, if an unnecessary frequency component is included in the driving waveform of the piezoelectric transformer, the element will be subjected to unnecessary vibration, which results in deterioration of the reliability.
A conventional method and apparatus for driving the piezoelectric transformer disclosed in Japanese Patent Application, First Publication No. Hei 9-135573 controls the load current flowing in a cold cathode fluorescent lamp as a load at a fixed level irrespective of the ambient circumstances. The above mentioned conventional method first detects the phase difference between the input and output powers by a phase difference detection circuit 78 and, while maintaining the pulse width of the driving signal variable, restricts the fluctuating frequency width of the driving frequency by the phase difference. However, the above method is not effective when the natural value is included within a frequency width restricted by the phase difference.
It is therefore the object of the present invention to provide a method and apparatus for driving a piezoelectric transformer for a high voltage power supply so as to be capable of expanding a variable range of the outputting voltage.
Another object of the present invention is to provide a method and apparatus for driving a piezoelectric transformer so as to be capable of maintaining the reliability in the wide range of the source voltage.